1. Field of the Invention
This invention relates to new forms of semiconducting copper aluminum sulfide and their preparation.
2. Description of the Prior Art
Many compounds of type ABX.sub.2 wherein A is a monovalent metal, B a trivalent metal, and X a chalcogen are known. In 1953 H. Hahn et al., Z Anorg. Allg. Chem. 271, 153 (1953) prepared the compounds CuAlS.sub.2 and CuAlTe.sub.2 among others and found that they had chalcopyrite-type structures and the space group I42d. The compounds were obtained by heating mixtures of CuX with the stoichiometrically required quantities of aluminum and chalcogen, and their homogeneity range seemed to be limited to stoichiometric compositions, C. H. L. Goodman and R. W. Douglas, Physica 20, 1107 (1954) noted structural analogy to chalcopyrite, CuFeS.sub.2, and suggested that the compounds should be semiconductors; the energy gap of CuAlS.sub.2 was calculated. C. H. L. Goodman, U.S. Pat. No. 2,814,004 (1957), prepared polycrystalline, semiconducting chalcogenides of formula ABX.sub.2 by melting together in an inert atmosphere the theoretically required quantities of at least one of copper and silver with at least one of aluminum, gallium, indium, and thallium, and at least one of sulfur, selenium, and tellurium.
In 1969 W. N. Honeyman reported, J. Phys. Chem. Solids 30, 1935 (1969), growth of single crystals of semiconducting CuAlS.sub.2 by vapor transport of the polycrystalline substances with iodine in vacuo in sealed tubes. Crystals of CuAlS.sub.2 were dark green to black in color with an energy gap of 3.35 eV, a resistivity of 10.sup.5 to 10.sup.7 ohm-cm., and a negative Seebeck coefficient indicating n-type conductivity.